How to show the panel with Geometry - Modification - Offset / Extrude
ah found it, partially through here:

https://spolearninglab.com/curriculum/software/3d_modeling/blender/blender_02.html



You simply have to click on that icon with two white dots connected with a curved line (have no idea how its called, when I do a mouseover over it, I get:

Type of active data to display and edit
Data: Object data
Python: SpaceProperties.context
bpy.Data.screens("Default") ... context


... which tells me absolutely nothing - but by the icon image, I'd guess this submenu is called "Curves"); and then you get Geometry panel...